Screw compressors in refrigeration and air conditioning applications commonly have slide valves which are adjusted to control capacity. When the capacity is too low, oil is "lost" to the evaporator with the potential for damage to the compressor due to inadequate lubrication. At light load the sensing of the slide valve position is not good enough for proper control since the actual capacity is a function of the discharge pressure. Specifically, there is a high leakback of refrigerant and oil at high discharge pressure so the capacity is less at high head then at low head for the same slide valve position.